The present invention relates to an electrostatic ink jet head which controls ejection of ink by use of electrostatic force.
An electrostatic ejection ink jet recording method is a method of recording an image corresponding to image data on a recording medium, in which ink containing an charged fine particle component is used, a predetermined voltage is applied to each individual electrode unit of an ink jet head according to the image data, and thus ejection of the ink is controlled by use of electrostatic force. As a recording apparatus which adopts this electrostatic ejection ink jet recording method, for example, an ink jet recording apparatus as disclosed in JP 10-230608 A is known.
FIG. 7 is a conceptual view showing an example of a schematic configuration of an ink jet head of the ink jet recording apparatus as disclosed in the above-described publication. In an ink jet head 70 shown in FIG. 7, conceptually shown is only one individual electrode unit constituting the ink jet head of the ink jet recording apparatus as disclosed in the above-described publication, and the ink jet head 70 includes a head substrate 12, an ink guide 14, an insulating substrate 16, a drive electrode 72, and an opposing electrode 22.
The ink guide 14 is disposed on the head substrate 12, and in a center portion of the ink guide 14, a notch which serves as an ink guide groove 26 is formed in a vertical direction of the drawing. In the insulating substrate 16, a through-hole 28 is formed in a corresponding position where the ink guide 14 is disposed. The ink guide 14 extends through the through-hole 28 formed in the insulating substrate 16, and a tip end portion of the ink guide 14 protrudes upward from a surface of the insulating substrate 16, which is an upper surface in the drawing.
The drive electrode 72 is provided in a ring shape for each individual electrode unit on the upper surface of the insulating substrate 16 in the drawing so as to surround a periphery of the through-hole 28 formed in the insulating substrate 16. The head substrate 12 and the insulating substrate 16 are arranged apart from each other at a predetermined interval, and an ink flow path 30 is formed therebetween. The opposing electrode 22 is disposed at a position opposite to the tip end portion of the ink guide 14, and a recording medium P is disposed on a surface of the opposing electrode 22, which is a lower surface in the drawing.
FIG. 8 is a conceptual view showing a configuration example of a driver for the drive electrode. A driver 80 shown in FIG. 8 includes an FET (field effect transistor) 74 and resistor elements 76 and 78. A drain of the FET 74 is connected to the drive electrode 72, a source thereof is connected to the ground, and to a gate thereof, a control signal is inputted. The resistor element 76 is connected between a high-voltage power source and the drive electrode 72, and the resistor element 78 is connected between the input for the control signal and the ground.
In the driver 80, the control signal is switched to a high level or a low level according to the image data. When the control signal is switched to the high level, the FET 74 is turned on, and the drive electrode 72 is switched to a ground level. On the other hand, when the control signal is switched to the low level, the FET 74 is turned off, and the drive electrode 72 is switched to a high-voltage level of the high-voltage power source. Specifically, the drive electrode 72 is frequently switched between the ground level and the high-voltage level according to the image data (control signal).
When performing the recording, from a right side to a left side in FIG. 7, ink containing the fine particle component charged to the same polarity as that of a high voltage applied to the drive electrode 72 is circulated.
In a state where the drive electrode 72 is at the ground level, a field intensity in the vicinity of the tip end portion of the ink guide 14 is low, and the ink is not ejected from the tip end portion of the ink guide 14. In this case, a part of the ink rises along the ink guide groove 26 formed in the ink guide 14 due to capillarity, and rises above the upper surface of the insulating substrate 16 in the drawing.
On the other hand, when the high voltage is applied to the drive electrode 72, the ink, which has risen along the ink guide groove 26 of the ink guide 14 and has risen above the upper surface of the insulating substrate 16 in the drawing, is ejected from the tip end portion of the ink guide 14 due to repulsive force and is attracted by the opposing electrode 22 biased to a negative voltage, to thereby adhere onto the recording medium P.
In such a manner, the recording is performed while relatively moving the ink jet head 70 and the recording medium P disposed on the opposing electrode 22, and thus the image corresponding to the image data is recorded on the recording medium P.
Incidentally, in the case of a recording apparatus for which high definition and high speed are required, necessarily, a line head capable of recording an image simultaneously for one line will be required. For example, in the case of a recording apparatus with specifications of 1200 dpi (dot per inch) and 60 ppm (page per minute), in a line head capable of recording an image on a recording medium with a width of 10 inches, an enormous number of individual electrode units, i.e., 12,000 electrodes equivalent to the number of pixels for one line, and the same number of drive circuits for driving the respective individual electrode units, are arranged.
In this case, it is necessary that the individual electrode units and the drive circuits be packaged in the line head with extremely high density from a physical viewpoint in a direction of the line. The drive circuits use a high voltage, for example, of approximately 600 V, and accordingly, when the individual electrode units and the drive circuits are arranged with high density, a risk of an electrical discharge is increased. Hence, it is extremely difficult to attain both the high-density packaging and the high voltage.
Moreover, in the above-described drive circuits, when it is assumed that a current of 1 mA is caused to flow per individual electrode unit, a current of 12 A is caused to flow at the maximum through 12,000 individual electrode units. Hence, when the voltage to be switched is 600 V, power consumption reaches 7.2 kW. Even if efficiency of the high-voltage power source is 100%, a power source of 36 A under AC 200 V will be required. Still, only a single color image can be recorded on a recording medium of the A4 size, and this is too impractical in terms of a system.